In ultrasonic imaging systems utilizing a phased array of transducer elements, pulses of ultrasonic waves are successively transmitted along different radial lines having their origin in the center of the array. When a pulse traveling along a radial line meets body tissue, a portion of its energy is reflected back to the array, but because the distances between the point of reflection and each of the transducers is different, the electrical waves produced by the transducers in response to the reflection have different phases. Summing these electrical waves would produce a weak signal for the purpose of controlling the intensity of an image. In order to obtain a strong signal, the electrical waves must be brought reasonably close to a cophasal relationship. This is not done at all points along a radial line but at each of a plurality of what are called "focal points". Best focus is attained at these points but if they are close enough together, the worst focus at points between them is tolerable. The distance between the points of worst focus on either side of a focal point is called a "focal zone".
In attaining the desired cophasal relationship, it is necessary to compensate for the respective differences in the time it takes the reflections from a focal point to travel the different distances to the respective transducer elements. Compensation is attained by introducing the proper effective compensating delays for each focal point into the paths of the electrical waves for each transducer element. Initially, the required information was burned into ROMs that were read at the appropriate times and used to control the means for providing the necessary delays. As this necessitated a large ROM capacity, microprocessors were used to provide in real time the information as to the difference in the time required for a reflection to travel from each focal point to each transducer. Whereas this method was satisfactory for systems having 64 transducers, 16 focal zones and 128 radial lines, it is impracticable for system in which the numbers of these parameters are substantially increased. If, for example, a system has 128 transducers, 256 radial lines and requires many more focal zones because of the loss in depth of field resulting from the increased aperture, the magnitude of the task of making the calculations increases by a factor of about 8.